1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 7-197837 discloses an air-fuel ratio control device, for an engine, which is provided with a three way catalyst arranged in an exhaust passage of the engine, an upstream-side of air-fuel ratio sensor arranged in the exhaust passage upstream of the three way catalyst, and an downstream-side air-fuel ratio sensor arranged in the exhaust passage downstream of the three way catalyst. In the device, a parameter for controlling the air-fuel ratio is calculated on the basis of the output of the upstream-side sensor, the parameter is corrected by a correction coefficient which is calculated on the basis of the output of the downstream-side sensor, and the air-fuel ratio is controlled to be a target air-fuel ratio using the corrected parameter. The correction coefficient is calculated by PID control, i.e., on the basis of a deviation between the output of the downstream-side sensor and the value corresponding to the target air-fuel ratio (the proportional term), an integral value of the deviation (the integral term), and a differential value of the deviation (the differential term).
When the target air-fuel ratio is the stoichiometric air-fuel ratio, a Z-output type oxygen concentration sensor, of which the output changes stepwise around the stoichiometric air-fuel ratio, is used as the downstream-side sensor. However, an air-fuel ratio region where the output of the downstream-side sensor corresponds, one to one, to the air-fuel ratio, is a very narrow region which is around the stoichiometric air-fuel ratio. Further, in this air-fuel ratio region, even when the air-fuel ratio changes slightly, the output of the downstream-side sensor changes widely. Thus, the differential term will be unstable. Accordingly, in this case, the air-fuel ratio can be controlled accurately if the differential term is not used, i.e., if the air-fuel ratio is not controlled by the change in the output of the downstream-side sensor with time.
On the other hand, the three way catalyst has an oxygen storage capacity, and thus the change in the air-fuel ratio downstream of the three way catalyst is more moderate than that upstream of the three way catalyst. Therefore, when the output of the downstream-side sensor changes suddenly toward, for example, the lean side, it is found that the air-fuel ratio upstream of the three way catalyst changes very suddenly toward the lean side. However, if the air-fuel ratio changes very suddenly toward the lean side, the NO.sub.x purification ratio of the three way catalyst may deteriorate, and thus a large amount of NO.sub.x may be discharged from the three way catalyst. Also, if the output of the downstream-side sensor changes suddenly toward the rich side, a large amount of HC or CO may be discharged from the three way catalyst. Note that, in this case, the amount of NO.sub.x, HC, or CO discharged from the three way catalyst becomes larger, as the changing rate of the output of the downstream-side sensor becomes larger. Accordingly, the air-fuel ratio is needed to be controlled on the basis of the change in the output of the downstream-side sensor with time, to purify the exhaust gas in the three way catalyst sufficiently. The publication mentioned above does not suggest the problem.